1. Field of the Invention
The present invention relates to a technique for generating a progressive video signal from an interlaced video signal and, in particular an image processing apparatus.
2. Description of the Related Art
As existing color television signal schemes, an NTSC (National Television System Committee) scheme, PAL (Phase Alternation by Line) scheme, and the like are known. These signal schemes are characterized by interlaced scan (a video signal based on such signal scheme is called an interlaced video signal).
Interlaced scan is also called interlace scan, and is a scheme for scanning by interlacing scan lines one by one. In case of an NTSC signal, since one image is expressed by 525 scan lines, it is decomposed into 262.5 lines for interlaced scan. Due to the presence of a fraction of 0.5 below the decimal point, when one scan is complete and another scan starts, the return position deviates by 0.5. As a result, picture elements which undergo the first scan and those which undergo the second scan deviate in position, and these picture elements are combined to display an image defined by 525 lines. At this time, an image defined by 262.5 scan lines is called a field, and an image defined by 525 lines which are transmitted by two vertical scans is called a frame. In this way, respective fields define images which alternately deviate from each other. In general, an even-numbered field is called an even field, and an odd-numbered field is called an odd field.
On the other hand, as a scan method which is different from interlaced scan schemes such as NTSC, PAL, and the like generally used in the color television signal schemes, progressive scan is known. A video signal in such signal scheme is called a progressive video signal. Progressive scan is also called sequential scan, and sequentially scans and displays an image. For example, image display panels such as a PDP (Plasma Display Panel), LCD (Liquid Crystal Display), LED (Light Emitting Diode), and the like make progressive scan.
In order to display a color television signal on the PDP, LCD, LED, or the like, processing for converting an interlaced scan signal into a progressive scan signal is required. This processing is generally called IP conversion (Interlaced to Progressive conversion) processing.
Various IP conversion processing schemes are known. In particular, recently, a motion adaptive IP conversion schemes which detects a motion of an image based on differences of pixel data between fields and adaptively generates line data in accordance with a moving image or still image is popularly used to attain high image quality.
This scheme performs intrafield interpolation for a moving image, and interfield interpolation for a still image. More specifically, image data suited to a moving image is generated by interpolating an image in a field for which line data are to be generated, and image data suited to a still image is generated by interpolating images between two fields including a field for which line data are to be generated. The generated image data suited to a moving image will be referred to as moving image interpolation data hereinafter, and the image data suited to a still image will be referred to as still image interpolation data hereinafter. In the motion adaptive IP conversion scheme, the moving image interpolation data and still image interpolation data are adaptively mixed based on changes of pixels included in a field, thus generating image data of a new line.
The IP conversion scheme which generates image data by mixing the moving image and still image interpolation data can be classified into field difference motion adaptive IP conversion and frame difference motion adaptive IP conversion depending on information used to determine the mixing ratio between the moving image interpolation data and still image interpolation data.
The field difference motion adaptive IP conversion is a scheme for determining the mixing ratio based on the difference between a field including a pixel to be interpolated and a field before or after that field (Japanese Patent Laid-Open No. 2006-41619). With this method, if the difference is large, a high mixing ratio of the moving image interpolation data is set. If the difference is small, a high mixing ratio of the still image interpolation data is set.
By contrast, the frame difference motion adaptive IP conversion is a scheme for determining the mixing ratio based on a frame difference of a field including a pixel to be interpolated and fields before and after that field (Japanese Patent Laid-Open No. 2002-185933). With this method as well, if the difference is large, a high mixing ratio of the moving image interpolation data is set. If the difference is small, a high mixing ratio of the still image interpolation data is set.
These field difference motion adaptive IP conversion and frame difference motion adaptive IP conversion have a merit of reducing the number of fields required to execute the IP conversion processing. That is, the number of fields required for the field difference motion adaptive IP conversion is two, and that required for the frame difference motion adaptive IP conversion is three. For this reason, upon implementing an IP conversion processing circuit, the field memory size can be reduced, and the circuit scale can be small, thus allowing implementation at low cost.
Since the field difference motion adaptive IP conversion is an algorithm using a field including a pixel to be interpolated and a field before or after that field, it has a higher time resolution than the frame difference motion adaptive IP conversion. For this reason, the field difference motion adaptive IP conversion can execute the IP conversion processing with high precision in the time direction as its characteristic feature.
In the motion adaptive IP conversion, when the ratio of the moving image interpolation data is higher than the still image interpolation data, since the moving image interpolation is processing in a single field, a frame does not suffer any serious collapse. By contrast, when a moving image is erroneously determined as a still image, a single frame is generated from two fields having different data according to a motion. As a result, the generated data suffers collapse as a picture (for example, the edge of an image is jagged, horizontal stripes stand out, or double images are seen in some cases). For this reason, the conventional motion adaptive IP conversion tends to set a higher ratio of the moving image interpolation data than the still image interpolation data.
The field difference motion adaptive IP conversion inherits such characteristics of the algorithm in the motion adaptive IP conversion, and further requires a special measure. In the field difference motion adaptive IP conversion, since one of fields, for which the difference is to be calculated, includes no spatially corresponding pixels, a difference value is calculated using tentative pixels generated using surrounding pixels of an interpolation position. Hence, the difference value readily becomes large, and in order to prevent collapse as a picture, the ratio of the moving image interpolation data tends to be set higher than the still image interpolation data. However, the converted image has a low vertical resolution unless a still image is correctly determined as a still image.
The existing IP conversion processing specifies the number of fields to be used, and it is difficult to optimize the IP conversion processing precision and the circuit scale upon implementing the algorithm of the IP conversion processing as a processing circuit. The precision of the IP conversion processing can be improved by increasing the number of fields used in determining the mixing ratio. However, when the number of fields to be used is increased, the circuit scale may increase upon implementing the processing circuit. It is not generally desirable since this leads to an increase in development cost of the processing circuit, and a physically large processing circuit size. For this reason, the number of fields to be used needs to be adaptively changed to optimize the IP conversion processing precision and circuit scale. However, the existing IP conversion processing specifies the number of fields to be used, thus disturbing optimization.
An increase in interfield difference data to be used basically improves the precision of the IP conversion processing. However, when fields before and after a field including a pixel to be interpolated are apparently different images, such increase may lower the precision. For example, in case of a scene change, the precision of the IP conversion processing lowers upon increasing the interfield difference data. When a scene change takes place in fields before and after a field including a pixel to be interpolated, it is not preferable to correct the mixing ratio.